(1) Field of the Invention
This invention relates to a bearing assembly with a seal structure including a ferrofluid.
(2) Description of the Prior Art
Referring to FIGS. 3 and 4, there are shown conventional bearing assemblies which are used, for example in a magnetic disc motor, and wherein a housing H receives a shaft S on which is disposed a bearing means B. Sealing means C and rotor R are fixed to an outer circumference of the rotor R. A pulley P is fixed to the shaft S and is driven by a belt (not illustrated). A dust prevention casing F is provided about the rotor R.
The bearing means B is comprised of an outer race 1, an inner race 2, a plurality of balls 3 disposed between the outer race 1 and the inner race 2 and a pair of cover members 4 for covering openings between the races 1 and 2 and a retainer D. A stopper ring 5 is mounted to the housing H.
As shown in FIG. 4, the seal means C includes a pair of pole pieces 6, 6, an annular magnet 7 disposed between the pair of pole pieces 6, 6 and a ferrofluid injected into gaps g formed between the pole pieces 6, 6 and the shaft S. A seal member 8 is comprised of pole pieces 6, 6 and the annular magnet 7. The seal member 8 is disposed at the rotor R side of the bearing means B by way of a non-magnetic spacer 9 disposed between a right side pole piece 6 and the outer race 1. The outer periphery of the seal member 8 is fixed within an inner periphery of the housing H.
With such construction, there is formed a magnetic circuit 11 between the pole pieces 6, 6 and the shaft S via the annular magnet 7. The ferrofluid 10 injected into the gaps g maintains a sufficient pressure resistance. As a result, an evaporated mist of a lubricant, such as, for example grease, in the bearing means B can be sealed completely in a manner to prevent leakage of the lubricant.
In the aforesaid conventional bearing assembly, the seal means C is of a three-pieced structure comprised of the pair of pole pieces 6, 6 and the annular magnet 7. Further, the seal means C is spaced apart from the bearing means B by the spacer 9. Accordingly, the total width of the two pole pieces 6, 6, the annular magnet and the spacer 9 is large, so that such a seal means is not suitable for thin application.
As hereinabove described, since the seal member 8 is of a three-pieced structure, fixation of the two pole pieces 6, 6 to the annular magnet 7 is cumbersome. In addition, there is the problem that it is difficult to obtain exact concentricity of each pole piece. As a result, when fixing the pole pieces 6, 6 within the housing H, the gap g between each pole piece 6, 6 and the shaft S becomes irregular thereby resulting in irregular pressure resistance of the ferrofluid retained in the gaps. Consequently, pressure resistance is partially decreased and durability of the seal means may deteriorate.